We often think of watts as a measurement of electricity when in fact it is a measurement of power. There is a very direct relationship between watts and horsepower that being 746 watts in a horsepower. So, if you need to know how much kW is required from a generator to run a motor of a particular horsepower simply multiply the horsepower rating of the motor by .746. Ex. 50hp times .746 equals 37.3kW. You then also need to add 10% to that figure to compensate for the fact that the motor being powered is not 100% efficient making the kW required to run the 50hp motor to be 42kW. So the equation to deduce kW to run a particular motor.  So the equation to deduce kW to run a particular motor is: motor hp x .85 = running kW.
You'll notice that 42kW is almost equal to 50hp. When sizing a generator, it's prudent to size the generator so that it never sees more than 80% of its capacity. The rule of thumb when sizing generators relative to running a motor(s) is to match the kW based on the horsepower of the motor(s) you are expecting to run. This allows for the additional "headroom" in the generator based on the motor horsepower it's running. Ex. (2) 50hp motors will require a 100kW generator.
Now that we've handled motor running, let's talk about motor starting. Starting a motor takes significantly more power than running a motor; typically somewhere in the neighborhood of 6 times as much power. This is based on the NEMA code and/or the Locked Rotor Amp (LRA) nameplate rating of the motor. These figures are utilized to deduce the starting kVA (skVA) requirement of the motor. Once this skVA requirement is deduced, it's a matter of matching it to a generator that has that much skVA capacity. The rule of thumb is; multiply the horsepower of the motor times 2.5 to get a generator with an adequately sized alternator to start that motor across the line. This rule of thumb will keep it at 30% voltage dip or less. Ex. a 50hp motor will require a 125kW generator to start it across the line.
In summary, the ratio of kW to HP in starting a motor is 2.5 to 1 and to run the motor is 1 to 1. If the motor is equipped with some type of soft-starter that ratio becomes 1 to 1 for both running and starting. We'll talk about soft-starters in some other installment.
Doran Carder
Application Specialist
GEN-TECH |